The invention relates to a method for determining rotor position for a brushless direct current (so-called BLDC) motor without using a position sensor. The determined position information can be applied to controlling the motor.
The control of BLDC motors includes an electronic commutator, which determines the necessary excitations of the windings of the different phases according to the actual position of the rotor. The classical way to determine rotor position is to use position sensors that trigger the switch from one commutation state to another. Using such position sensor mechanisms increases the price especially if the motor is located separately from the controlling electronics in the given application.
Rotor position determination without using special position sensor mechanism is based on the fact that a position can be obtained basically from the electromagnetic characteristics of the machine itself.
Maybe the most widely spread method is the back-EMF method, where the rotor position is determined on the basis of the induced voltage functions by rotation (Lenz's law). The disadvantage of this method is that the amplitude of the induced voltage is practically too low for determining the position at low rotational speed. So, this method cannot be used to start the motor.
During standstill and at low speed, typically, extra excitation is inserted to get information about the rotor position. One classification of these methods can be done along the property if they include excitation of only one phase or several phases of the motor for calculating the position estimate. The methods that excite several or all phases of the motor have the advantage that no initial information is necessary about the flux and/or inductivity characteristics of the motor.
The general pre-assumption is only that inductivity of the stator windings varies in the function of the position, and that its periodicity is 360° el (and it is not 180° el). Sufficient condition for this is if the inductivity of the windings of the phases enters into the nonlinear (saturation) region of the characteristics at least in a part of the position range.
European patent specification EP-0251785 proposes a method to determine commutation on the basis of the highest measured value among the responses for the excitation pulses. The excitation pulses are provided for each phase in both polarities and have the same fixed time period. Here, the estimated position data is selected out of a discrete set having a resulting of 180° el/p, where p denotes the number of stator phases.
European patent specification EP-0536113 bases its method also on 2p number of measurement data. Here, continuous position estimation is provided by calculating the phase of the first harmonic out of the inverse discrete Fourier transform of the response values. The calculated position information is used to determine starting commutation command. After this starting commutation command, it is proposed to control the change of the commutation with an experimentally predetermined series of commutation time periods until a limit speed is reached. After reaching the limit speed, the back-EMF method is applied for commutation control.
The basic idea described in WO-2003052919 is similar to that of EP-0251785. But here the maximal amplitude of the responses for all test pulses is fixed to the same value, and the necessary time periods to reach the limit are measured. Then, commutation is selected according to the phase excitation with the smallest time period. After initial measurement, it is further proposed here to reduce the number of measurement pulses to three if the direction of rotation is not known, and to two if the direction is known.
There is therefore needed a method to determine the rotor position through simple calculation using reduced number of excitations, but still resulting in a continuous estimate, that is, more advantageous than the above methods.
This need is met by a method of determining the angular position of the rotor of a brushless direct current motor having a stator with multi-phase windings, a control circuitry that provides excitations switched after one another to the motor phases such that excitations produce stator flux vectors lying in different directions in the full 360 electrical degree range, and a measurement circuitry that delivers responses for the phase excitations. The position is estimated by selecting the main excitation stator flux vector, which results in the minimal inductivity shown by the measured response, and selecting the two other vectors in the previous and subsequent directions with respect to the direction of the main vector, and by computing the ratio between differences of measured responses for the selected stator flux vectors.
As such, the method of the invention determines the maximum of the nonlinear curve of the counterinductivity characteristics out of the small number of measured data. The displacement of the maximum of the curve is equivalent to the angular displacement of the rotor from the position, where rotor magnetic polarity is aligned with the direction of the 1st stator flux vector.
Accordingly, the rotor position is determined by simple calculation using reduced number of excitations resulting in a continuous estimate.
According to a further aspect of the invention, the motor works in the non-linear (saturation) region of the inductivity characteristics at least in a part of the position range.
According to a further aspect of the invention, the measured responses of the stator phases are proportional to the actual inductivity or counterinductivity of the motor windings. A measured response will be, for example, proportional to the inductivity if the time period of the response signal transient is measured until reaches a fixed maximum level. On the other hand, a measured response will be, for example, proportional to the actual counterinductivity if the maximum of the current response transient is measured for an excitation pulse having a fixed time period.
According to a further aspect of the invention, the motor phases are excited with at least 3 stator flux vectors selected from the 2p stator flux vectors lying at electrical angles of k=0, . . . , 2p−1, where p denotes the number of phases of the motor. Selection of the excitation directions is made in such a way that at least the main vector and the vectors lying in the previous and subsequent directions are chosen for excitation.
According to a further aspect of the invention, the position dependent motor values are inverted before being applied in the further calculation, if they are proportional to the inductivity of the windings. Thus, a data set with values proportional to counterinductivity of the windings is always used in the position estimation. The measured or calculated position dependent motor values that are proportional to the counterinductivity are denoted by {i0, . . . , i2p−1}
According to a further aspect of the invention, the selection of the main excitation stator flux vector is done by selecting the vector that corresponds to the maximal value among {i0, . . . , i2p−1}. If more than one equivalent maximal values are found, then any of their indices can be chosen for the index of the main excitation stator flux vector. Then, the ratio between differences of measured responses for the selected stator flux vectors is computed as a fraction, the numerator of which is in minus iλ, and the denominator of which is im minus in, if iλ is greater than in, else if im does not equal iλ, then the denominator is im minus iλ, else the denominator is 1, where m denotes the index of the direction of the main excitation, and λ, n denote the indices of the directions before and, respectively, after the direction of the main excitation. Finally, the estimated position is obtained as m times π over 2p.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.